1993 Lumina coolant issue

I have a 1993 Lumina 3.1L, with 198,000 miles. I just drove about 50 miles back to school from home, and everything was fine. About 20 minutes later, I begin backing the car out of the driveway when I see a large puddle of coolant, and more dripping from somewhere in the rear of the engine. There was also some coolant on the timing belt, and the reservoir was completely filled with light-colored gunk. About 6 months ago the heater hose rusted out and was replaced, and the system was flushed after the mechanic noticed the radiator had lots of corrosion built up. I also believe the radiator hoses were replaced too, so I am completely stumped as to what might have happened, since it was fine when I left home, and an hour and a half later it was emptying out. Thanks for any input you may have, I'd greatly appreciate it.


P.S. I started the car again this morning, filled it with water and it seemed as though the water pump was leaking, because the wheel that the belt drives was spraying coolant at me, which also explains why the belt had coolant on it. I still have no idea why some of the coolant solidified in the reservoir (there's also a few grainy bits in the radiator). The car never overheated last night or this morning, and the radiator is full, if that helps.